Finishing Treatment

Sealing. Thermal spray coatings usually have a structure with inherent porosity that ranges from less than 2 to more than 15 vol%, depending on the process by which the coating is deposited and the material sprayed. At least some of this porosity is interconnected. In many applications, coatings are exposed to corrosive fluids (liquids or gases) or hydraulic fluids that can infiltrate the pores, resulting in fluid leakage or corrosion throughout the coating or of the base material. These conditions can contribute to the premature failure of the coating. Many such applications, therefore, require the coating to be sealed before finishing. Sealing a coating may also help to reduce particle pullout from the surface during finishing for coatings with low cohesive strength.

To ensure as complete a sealing of the coating as possible, it is necessary to apply the sealant material as soon after coating as possible and prior to surface finishing. Sealant materials such as waxes, epoxies, phenolics, and inorganics are readily available and easily applied. The wax sealants are useful in preventing infiltration of liquids at low service temperatures. Resin-based sealants may be effective at temperatures up to about 260 °C (500 °F). Some silicone-based sealants have been reported to provide effective protection in salt spray tests conducted in accordance with military standards up to 480 °C (900 °F). Epoxy and phenolic sealants are usually more effective on coatings with higher porosity within their limits of stability (up to about 300 °C, or 570 °F).

One of the most effective methods of sealing coating porosity is vacuum impregnation. This method will usually fill all interconnected pores open to the exterior surface. To vacuum impregnate, the part is immersed in the sealant and placed in a vacuum chamber, and a soft vacuum is drawn. When the vacuum is released, air pressure forces the sealant into the pores. Most applications do not require this procedure, however. Low-viscosity anaerobic sealers may also be particularly penetrating. The depth of penetration of some sealants may exceed 1.8 mm (0.070 in.). Regardless of the method or type of sealant used, pores or interconnected channels that are not connected to the exterior surface cannot be sealed and machining or wear in service may open these with consequent loss of corrosion protection.

Coating Finishing. Although thermal spray coatings are used with their surfaces in the as-deposited condition for some applications, these surfaces are too rough for most service conditions. Therefore they are usually finished by methods such as grinding, lapping, polishing, machining, abrasive brushing, or vibratory finishing. Although the techniques are common to those used for finishing solid metallics and ceramics, great care must be taken not to damage the coatings, causing excessive surface porosity due to pullout of coating particles or cracking due to thermal stresses. The ultimate surface finish that can be achieved with a thermal spray coating is a function not only of its composition, but also of the deposition parameters used to produce it, because they are largely responsible for the amount and size of the true porosity in the coating and the cohesive strength or particle-to-particle bonding within the coating. The best finish that can be achieved may vary, therefore, from a matte surface with a roughness of about 1 pm (40 pin.) Ra and pits exceeding 0.05 mm (0.002 in.) in diameter for a flame-sprayed coating to a virtually pit-free mirror finish with a roughness of less than 0.025 pm (1 pin.) Ra for some very-high-velocity coatings.

If a coating is to be sealed, the sealing should be done before any finishing operation. It is extremely difficult to remove finishing fluids and debris from an unsealed surface, and these will interfere with the sealing. Sealing may also help to prevent the embedment of finishing debris in a surface, which would cause abrasive wear in service.

Some of the softer metallic coatings can be machined with single-point high-speed tool steels. Better surface finishes can be achieved with carbide or coated carbide tools. Table 4 includes typical parameters for machining some classes of metallic coatings. Usually, lower infeeds are used than with wrought materials. Figure 10 shows the configuration of typical carbide and steel tools. Burnishing is occasionally used with soft materials such as tin, zinc, and babbitt to produce a smooth, dense bearing surface.

Cermet and ceramic coatings require grinding, and many metallic coatings can be more effectively ground than singlepoint machined. Some coatings can be ground with oxide or silicon carbide wheels, but cubic boron nitride or diamond wheels may be necessary for some of the hardest coatings, and they are frequently more cost-effective and produce better finishes for many other coatings. Specific grinding wheel selection is important and varies with the coating composition and type. It is probably best to consult with wheel manufacturers for specific coatings. Some guidelines for diamond grinding:

11. Visually compare the part at 50* with a control sample of known quality.

Regardless of the type of grinding wheel used, the wheel should be dressed frequently enough, and operating parameters should be chosen, to ensure clean cutting of the coating. Sparkout passes (passes with low contact pressure run until virtually no contact is being made) should never be used. The smeared material created by such a procedure can be easily dislodged in service and cause abrasive wear and other problems.

If grinding does not produce a sufficiently smooth surface, it may be necessary to lap the coating after grinding. Again, it is advisable to consult the manufacturers of lapping materials for specific recommendations. Some guidelines:

9. Clean parts after grinding and between changes to different-grade diamond laps--use ultrasonic cleaning if possible.

10. Visually compare the part at 50* with a control sample of known quality.

In addition to the traditional finishing techniques discussed above, a variety of other methods have been developed, particularly for nondimensional finishing. These include various abrasive brushes, belt grinding, "super" finishing, peening, and vibratory techniques. The use of nondimensional finishing is usually possible only when the dimensional specifications for the part are very loose, or when the part can be precisely and accurately preground and the deposition thickness and other characteristics such as waviness can be tightly controlled.

Coating Repair. The repair of thermal spray coatings by coating over service-worn or in-process damaged coatings is not generally recommended, even if the predeposited coating is reference ground, cleaned, and grit blasted. Adequate bond strength between the coating layers is seldom achieved, and there are no reliable nondestructive test techniques currently available to verify an adequately bonded interface. Therefore, the preferred procedure is to strip the existing coating and apply a completely new coating. Note that when applying a multilayered coating, it is best to apply each new layer over the as-deposited surface of the previous layer, not to grind and grit blast between layers.